The present invention relates to an epoxy resin composition suitable for encapsulation of semiconductor devices, or, more particularly, to an epoxy resin-based composition suitable for encapsulation of various kinds of semiconductor devices such as ICs, LSIs, VLSIs and the like exhibiting excellent resistance against moisture, high dimensional stability and low mechanical stress.
Various kinds of synthetic resin compositions are known and used in the prior art as a material for the encapsulation of semiconductor devices including silicone resins, diallyl phthalate resins, phenolic resins, polyphenylene sulfide resins as the principal component thereof. Among these conventional resin compositions, epoxy resin-based compositions are currently used most widely and in the largest quantity in view of the excellent characteristics thereof such as the mechanical and electrical properties, heat resistance, adhesiveness, workability in molding and so on.
These epoxy resin-based compositions, however, are not free from several problems and disadvantages. Along with the trend in recent years toward further and fruther decreased thickness and size of electronic parts to facilitate a compact design of electronic instruments and larger and larger size of semiconductor devices to meet the requirement for an increased density of integration thereon, for example, problems are involved more and more in the undesirable phenomena of crack formation in the resin encapsulation and silicon chips, passivation cracks, mobility of aluminum wirings and so on. Accordingly, the resin compositions used for encapsulation of semiconductor devices should satisfy all of the requirements in respect of the stability against crack formation, moisture resistance, low coefficient of thermal expansion, low elastic modulus, high glass transition point and the like in order that the encapsulated semiconductor devices can withstand severe heat cycles and mechanical shocks. None of the conventional resin compositions for semiconductor encapsulation, however, is satisfactory in one or more respects.
While most of the epoxy resin-based compositions currently used for encapsulation of semiconductor devices are prepared by compounding a novolac-type epoxy resin with a large amount of an amorphous or crystalline silica filler, a crosslinking agent such as a phenol novolac and curing catalyst to promote curing by heating, any improvement in one respect is always achieved at the sacrifice of one or more of other properties so that the above mentioned requirements cannot be satisfied altogether at one time.
The inventors have previously proposed an epoxy resin-based composition for semiconductor encapsulation (see Japanese Patent Kokai No. 56-129246) by compounding an epoxy resin with an organopolysiloxane represented by the general formula [(--SiR.sub.2 --Z--SiR.sub.2 --O--).sub.a (--SiR.sub.2 --O--).sub.b ].sub.n, in which R is a substituted or unsubstitued monovalent hydrocarbon group, Z is a divalent organic group, n is a positive number having an average value of at least 10 and a and b are each zero or a positive number with the proviso that a +b =1. A problem in such a composition is that no highly uniform dispersion can be obtained in the "sea-and-islands" texture of the epoxy resin and the organopolysiloxane and further the structure formed by the dispersion is sometimes unstable so that the encapsulation formed of such a composition may be subject to decrease in the mechanical strengths and increase in the moisture permeability in addition to the problem of eventual bleeding of the organopolysiloxane ingredient on the surface of the cured resin composition.
It is also disclosed in Japanese Patent Publication 60-11973 that the above mentioned epoxy resin-based composition can be improved in respect of the mold-releasability without contamination of the metal mold when the organopolysiloxane compounded with an epoxy resin is modified with certain functional groups such as epoxy groups, amino groups and the like. The epoxy resin-based composition of this type is advantageous in respect of the low mechanical stress and decreased bleeding of the organopolysiloxane by virtue of the functional groups thereof having reactivity with the epoxy resin. The problem of low dispersibility insufficient to give a uniform "sea-and-islands" texture remains still unsolved even in such a resin composition so that the encapsulation formed by curing the composition is also disadvantageous in the low mechanical strengths.
The inventors have further proposed an epoxy resin-based composition for semiconductor encapsulation highly resistant against crack formation (see Japanese Patent Kokai No. 58-21417) in which an epoxy resin is compounded with, in place of an organopolysiloxane, a block copolymer composed of the segments of a polymeric aromatic moiety and segments of an organopolysiloxane moiety. Although such a resin composition is greatly improved in respect of the resistance against crack formation as a result of the decrease in the stress obtained by the addition of the block copolymer compatible with the epoxy resin, the resistance is still insufficient under extremely severe conditions, presumably, due to the components being so compatible that they hardly form a "sea-and-islands" texture.